The present invention generally relates to an apparatus for reading, recording and erasing minute data recorded on concentric or helical tracks of a disk by converging light thereon and more particularly, to a lens actuator for use in the apparatus.
For example, an audio data recording medium, a video disk and an optical disk are known as a data recording medium in which data is recorded on tracks. Data tracks on these data recording media are arranged concentrically or helically. It is known that an apparatus for reading such data optically includes an optical system for irradiating a beam spot on the data recording medium and a position detecting means for detecting relative position between the data tracks and the beam spot on the data recording medium.
FIG. 9 shows conceptually a conventional optical reader for reading the audio data recording medium, the video disk and the optical disk. A beam emitted from a laser light source 3 is converged on a data recording medium 1 through a collimating lens 4, a reflecting mirror 5 and an objective lens 2. A substrate of the data recording medium 1 has a thickness of 1.2 mm. The objective lens 2 is arranged to converge the beam on a recording layer of the data recording medium 1 through the substrate of 1.2 mm in thickness. Intensity of the beam reflected from the data recording medium 1 varies according to data of the data recording medium 1. Thus, by detecting intensity of the beam by a photodetector 7 via a detecting lens 6, the data recorded on the data recording medium 1 can be obtained as an electrical output.
Meanwhile, a data recording medium for high-density recording is newly proposed and has a substrate of 0.6 mm in thickness. Therefore, it is necessary for this data recording medium to provide another objective lens for converging the beam on a recording layer through the substrate of 0.6 mm in thickness.
FIG. 10 shows a known lens actuator utilizing the two kinds of the objective lenses through their changeover. In this known lens actuator, an objective lens 102 for the substrate of 1.2 mm in thickness and an objective lens 108 for the substrate of 0.6 mm in thickness are mounted on a base 110. For changeover of the objective lenses 102 and 108, the objective lenses 102 and 108 are rotated relative to a supporting shaft 109 so as to come to a center of the beam from the laser light source. A magnetic circuit for performing changeover of the objective lenses 102 and 108 and tracking follow-up of the data recording medium includes two pairs of driving coils 111 attached to the base 110, two pairs of iron pieces 112 fixed to the base 110 and a pair of magnets 113. The objective lenses 102 and 108 are positioned through their changeover by the iron pieces 122 and the magnets 113, while tracking is performed by the driving coils 111. At the time changeover between the objective lenses 102 and 108 is performed, large pulsating current is applied to the driving coils 111. Thus, it is possible to perform reproduction, recording and erasure on the data recording media having the substrates of different thicknesses.
In this known lens actuator of FIG. 10, the following problems arise. Since each of data tracks recorded on the data recording medium has a width of not more than 1 .mu.m and an interval of the tracks is about 1 .mu.m, control target of focusing of the objective lenses should be set at not more than .+-.0.1 82 m and tracking control in which the beam spot follows up the tracks should be set at not more than +0.1 .mu.m. Hence, it is quite essential that a movable member including the objective lenses not only is compact and light but has a balanced configuration such that an optical axis of each of the objective lenses is not inclined when the movable member is driven.
Furthermore, when focusing and tracking are performed on the data recording media having the substrates of different thicknesses, a plurality of the objective lenses should be mounted on the single movable member. Accordingly, conventionally, it has been extremely difficult to optimize focusing and tracking control such that a plurality of the objective lenses are accurately driven independently of one another.
Meanwhile, since tracking control and changeover of the objective lenses in accordance with the thicknesses of the substrates of the data recording media have an identical driving direction and are, respectively, provided with separate magnetic arrangements, the lens actuator and its control circuit become complicated structurally. Therefore, it has been quite difficult to simplify construction of a lens actuator which has functions of identifying an objective lens proper for the data recording medium and effecting changeover between the objective lenses and is capable of performing focusing and tracking control accurately.
Another known lens actuator utilizing the two kinds of the objective lenses through their changeover is shown in FIG. 11. In this known lens actuator, an objective lens 202 for the substrate of 1.2 mm in thickness and an objective lens 208 for the substrate of 0.6 mm in thickness are mounted on a base 210. For changeover of the objective lenses 202 and 208, the objective lenses 202 and 208 are rotated relative to a supporting shaft 209 so as to come to a center of the beam from the laser light source. A magnetic drive mechanism for performing changeover of the objective lenses 202 and 208 and tracking follow-up and focusing follow-up of the data recording medium is constituted by a pair of focusing magnets 214 subjected to bipolar magnetization vertically as shown in FIG. 12, a pair of tracking magnets 213 subjected to bipolar magnetization laterally and a pair of coils 221 and a pair of coils 222 which are mounted on the base 210 so as to confront the focusing magnets 214 and the tracking magnets 213, respectively. Furthermore, the objective lenses 202 and 208 are positioned in a tracking direction by four iron pieces 212.
When the coils 221 confront the focusing magnets 214, respectively as shown in FIG. 11, the coils 221 function as focusing drive coils. At this time, since the coils 222 confront the tracking magnets 213, respectively, the coils 222 function as tracking drive coils. If the coils 221 confront the tracking magnets 213, respectively upon changeover between the objective lenses 202 and 208, the coils 221 function as the tracking drive coils. Therefore, the two tracking magnets 213 are provided for the two objective lenses 202 and 208.
In this known arrangement of FIG. 11, the following inconveniences are incurred. Since driving directions of the coils 221 and 222 are switched between focusing direction and tracking direction upon changeover between the objective lenses 202 and 208, it is necessary to judge which one of the objective lenses 202 and 208 is being controlled. Furthermore, each time changeover between the objective lenses 202 and 208 is performed, it is necessary to electrically switch between focusing control and tracking control driving current flowing through the coils 221 and 222. Therefore, configuration of an electric circuit of the known lens actuator of FIG. 11 becomes extremely complicated and thus, control of the known lens actuator of FIG. 11 is quite difficult.